text: For meta-directing groups (electron withdrawing group or EWG), σ and σ are more positive than σ’. (The superscript, c, in table denotes data from Hammett, 1940.) For ortho-para directing groups (electron donating group or EDG), σ’ more positive than σ and σ. The difference between σ and σ’ (σ – σ’) is greater than that between σ and σ’(σ − σ’). This is expected as electron resonance effects are felt more strongly at the p-positions. The (σ – σ’) values can be taken as a reasonable measurement of the resonance effects.